Hi bhuang,

There are 6 odd numbers and 5 even numbers

Odd+Odd=Even and Even+Even=Even

there are 25 pairs of numbers that add to an even result.

The total number of pairs possible is

Therefore the probability of an even sum is

To answer the question....

Given that the sum is Even, which means one of the 25 selections leading to an even result occurred,

in how many ways out of these 25 would the two numbers have been odd ?

Hence the probability is the fraction of that quantity out of the 25.